The present invention relates to a cathode supporting structure for a color cathode-ray tube in which the arranging directions of three cathodes corresponding to three colors, R, G and B, respectively, of an electron gun of an in-line type color cathode-ray tube are positively held in parallel with one another. The structure is suitable for automatic assembly of the electron gun.
In FIG. 1, reference numeral 1 designates a phase plate; 2 designates a phosphor screen; 4, 5 and 6 designate cathodes comprising an electron gun for emitting three colors, R, G and B, respectively; and 3 designates a supporting structure for said cathodes. Reference characters G.sub.1, G.sub.2, G.sub.3 and G.sub.4 designate a first, a second, a third and a fourth grid, respectively.
In the in-line type color cathode-ray tube (shown in FIG. 1), three electron beams of an electron gun held in parallel with one another within the tube have to be present within one and the same plane. In this case, the electron emitting directions from the cathodes are required to be parallel with one another at least in a section from the cathode to the first grid electrode, to which the end the cathodes have to be supported in the direction parallel with one another within one and the same plane.
The color cathode-ray tubes are produced on a large scale. Therefore, the work can be accomplished so that the electron gun may be assembled simply and easily and the members may be accurately held at a predetermined relative position.
Normally, three cathodes are supported in a direction parallel with one another within one plane. In accordance with the aforementioned conditions, for example, Japanese Patent Application Laid-Open No. 57(1982)-170434 discloses a cathode supporting structure for a color cathode-ray tube having a construction in which as shown in FIGS. 2a and 2b, outer peripheral portions of three metal pipes for inserting a cathode 23a, 23b and 23c disposed in parallel with one another within one plane are surrounded by a single metallic elliptic outer supporting frame 22. Powdery glass 24 is filled between said outer supporting frame 22 and the three metal pipes 23a, 23b and 23c for inserting a cathode, and the powdery glass 24 is subjected to fixing by hot melting while accurately holding the relative position of the aforementioned members. Automation of the assembling steps is relatively easy.
In the above-described prior art, and in consideration of reliability with respect to fixing between the metal members and glass after assembled, the coefficient of thermal expansion is set in the relationship of the outer supporting frame&gt;crystallized glass&gt;metal pipe. More specifically, nickel-iron alloy for hermetic sealing soft glass, crystallized hard glass powder with low melting point are subject to fixing by hot melting, and iron-nickel cobalt for hermetic sealing hard glass are selected for the outer supporting frame 22, the glass powder 24 and the metal pipes 23a, 23b and 23c, respectively, whereby the effect of a shrinkage fit is obtained between the respective members after the step of fixing by hot melting of the crystallized hard glass powder with a low melting point.
In the past, an axial length L.sub.3 of the metal pipes 23a to 23c of the outer supporting frame 22 is
more than twice of an axial length L.sub.2 of the metal pipe in the crystallized glass portion fixed by hot melting, and the object for firmly fixing the members from each other has been fully achieved by the aforesaid section of materials. However, after the cathode supporting structure is completed, one lengthwise end of the outer supporting frame 22 (corresponding to the upper end in FIG. 2b) registers with one end of the crystallized glass portion 24 (corresponding to the upper end shown). However, the length L.sub.3 of the outer supporting frame 22 is twice of the length L.sub.2 of the glass portion and the other end of the outer supporting frame 22 extends beyond the other end of the crystallized glass portion and at a position extending in excess of the height of the glass portion. Therefore, the local shrink-force of a part of non-sealing in metallic outer supporting frame of the metal outer supporting frame 22 (in which crystallized glass portion is not present) intensively acts so that ends 23a' and 23c' of two outer pipes 23a and 23c among three metal pipes 23a to 23c, said ends 23a' and 23c' being located on the side wherein the outer supporting frame 22 extends downwardly of the end of the glass portion, are moved toward the center pipe 23b, and as a result, the arranging directions of three metal pipes 23a to 23c became not parallel with one another as shown in FIG. 2c (P.sub.1 '&lt;P.sub.1, and P.sub.2 '&lt;P.sub.2).
The pitch between the three metal pipes corresponds to pitches P.sub.1 and P.sub.2 between three electron beams of the in-line type color cathode-ray tube, and an error of .+-. 0.05 mm or more is not allowed. On the other hand, the prior art has a problem in that only the stress deformation caused by a difference in the aforementioned coefficient of thermal expansion (as shown at P.sub.1 ' and P.sub.2 ' in FIG. 2c) results in an error in excess of the aforesaid allowable error.